Documentation

PLANAR_CODE is the most common encoding for planar graphs, and is
supported by various generation and visualisation tools. It is a
binary format and not intended to be human-readable.

The default encoding only supports graphs with <256 nodes, and
takes 2*|E|+|N|+1 bytes per graph.

Please note that PLANAR_CODE is not suitable for graphs with
multiple loops on vertices (multiple edges with distinct endpoints
however are catered for). As such, no guarantees are made about what
happens with multiple loops.